Just before the release of the IPCC’s AR6 WGIII report (Mitigation of Climate Change) Joeri Rogelj had a Carbon Brief guest post on how not to interpret the emission scenarios in the IPCC report. It might have been to try and pre-empt some of the simplistic narratives that some have promoted about emission scenarios, but I can’t be sure about that being the motivation.

The key point being made was that

Scenarios can be thought of as stories of what could happen in the future. What they are not, it is important to note, are forecasts or predictions for the future.

As the article goes on to say, scenarios are essentially “what if” thought experiments that help us to answer some questions about what might happen in the future. They do not, however, encompass all possible futures, and are not predictions of the future.

However, the issue of scenario plausibility has been a bit of a hot topic and is something I’ve been thinking about a bit myself. I haven’t managed to really draw any strong conclusions, but the beauty of a blog is that I can present some ideas that others can challenge/clarify in the comments.

I tend to think that scenarios should be plausible in the sense of not violating some fundamental laws of physics, or the essentials of chemistry. I don’t necessarily think they need to pass some kind of societal plausibility test, at least in the sense of them being what we will probably do. They could be “what ifs”, or “what could have been” or even “what might be”. Of course, I do think the motivations, and assumptions, should be made clear when communicating the results of any analysis based on a particular scenario, but I don’t think they have to pass some kind of societal plausibility test.

However, one of the key talking points about scenarios is whether or not their plausibility should be more explicit. I can see some merit to this, but I can also see why this may not be appropriate. If scenarios are “what if” thought experiments, rather than forecasts or predictions, then we should be cautious of suggesting that they are more the latter than the former.

There’s also the issue of independence. If the point of scenarios is to understand the impact of various possible futures, then assigning something like a probability to a scenario might then influence the outcome. For example, if we claim that a particular scenario is impossible, we may then either give up trying to achieve it, or assume that we no longer need to do anything to avoid it. In a sense, the probability of a scenario emerging in reality could be influenced by the probability assigned to that scenario, which would then undermine the intention of these being policy relevant, rather than policy prescriptive.

A potentially interesting issue, though, is the self-consistency of scenarios. In most cases a scenario is developed and then that scenario is used as input to some kind of model to assess the potential impact of that scenario. However, rarely do these models then include how these impacts might influence the scenario itself.

This can have a number of potential consequences. It could be that the impact of following a particular scenario could be so severe that it essentially precludes anything like that scenario from actually emerging. On the other hand, if we don’t consider how the impacts might influence the scenario itself, we might conclude that everything will still essentially be fine even under some extreme scenarios. So, I think it’s important to make clear that these scenarios are often not really self-consistent.

As usual, this has got rather long. I do think we should mostly treat scenarios as “what ifs” and should let society decide what to do, or not do, given what these scenarios might be suggesting. Of course, I do think we should be very clear about the motivations, and assumptions, behind these scenarios, but – IMO – we should be cautious about explicitly defining the plausibility of these scenarios. However, I would also be interested to know what others think.

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38 Responses to Scenarios

  1. I should probably clarify something. Given that we normally consider a small number of scenarios, this won’t come close to encompassing all possible scenarios. Hence, we will almost certainly not follow any of these scenarios in reality. So, when I talk about a scenario emerging, I just mean something close to that scenario, not precisely that scenario.

  2. russellseitz says:

    “Hence, we will almost certainly not follow any of these scenarios in reality.”

    The reality is that such scenarios began driving climate advocacy playbooks, story lines and style manuals years before The Guardian and The Nation Institute amplified them further by organizing Covering Climate Now.

  3. I tend to agree we should treat scenarios as ‘what ifs’, and not try to weight their probabilities. However, I do think it would be useful to be very clear about the scale of change implied e.g. ‘carbon emissions would have to reduce by x% by ddd date on this scenario, with carbon sinks continuing to provide yyy etc.’. The complication comes in the many different variables that can generate a scenrio, with multiple combinations giving rise to the same emissions scenario; if as I assume this means net emissions. For policy makers, understanding how – for example – electrification can yield reduced demand, might be one such variable. So if they can see how the ‘natural’ or forced obsolence in ICE cars (to EVs) and Gas Boilers (to heat pumps) is perfectly consistent with a particular emissions scenario, then that may build confidence in it. This might be a good thing, without putting a figure on how likely it is, enabling policy ‘judgments’ to be made.

  4. Richard,
    That’s a good point. Rather than trying to define the plausibility of the scenario itself, we could be stressing what it would probably take to follow one. Either in terms of what we might want to avoid doing when it comes to high emission pathways, or what we might need to do to follow a low emission one.

  5. russellseitz says:

    Ford’s launch of a line of electric pick-up trucks affords an opportunity to advance ICE obsolescence and energy efficiency without further aggravation of the culture wars.

    Giving buyers the option to adjust battery capacity to suit their needs would allow suburbanites to save money by opting for modest energy storage, while leaving heavy lifters on farms or in the construction business free to invest in as much torque as circumstances require.

  6. anoilman says:

    russellseitz: I just bought a new Ford plug in hybrid. My car was getting old and this was actually… available…

    This thing is great, I haven’t used gas for days. There worse fears than range anxiety. Like plugging a Ford into the power grid. (At least my garage is detached.)

    All daily commutes are easily handled by a car with a small battery. Its really convenient not having to go the gas station.

  7. wmconnolley says:

    > scenarios are essentially “what if” thought experiments
    That’s fine, but passes the buck along to someone else to assess probability, at least implicitly. Unless you’re going for total policy irrelevance, the scenarios need to have some level of probability.

  8. Probability? Heck, WMC, the first test is for viability.

  9. Chubbs says:

    Recognizing that future political, economic and technology change can’t be predicted very well, the most important characteristic of scenarios is spread, and ability to tie scenario differences to a something that informs.

    Business as usual, is a very low probability scenario. We are in a period of more rapid change vs recent decades. Not the least of the changes is climate. Though not reflected in any scenario, climate change will become increasingly important with time as a driver of human activity. Hopefully self-limiting, with more visible climate change producing stronger mitigation.

  10. Economic forecasting is game theory, which is intractable to model. Energy resource depletion forecasting is not game theory, as long as it doesn’t factor in economic forecasting. There lies the rub.

  11. Fergus Brown says:

    I don’t think the pathways/scenarios are ‘what ifs’ so much as simply ‘if thens’. According to the SPM they are internally consistent and contain plausible assumptions about societal actions. In response to your suggestions, how then to assign a probability to these? I’m inclined to think that however it is done there will be a degree of ‘loading’ regardless, and could result in more arguments about whether it’s a duck or not, rather than action plans, never mind actions. We have gone past the stage where we should be justifying policy or action choices beyond the very simple “does it reduce emissions or add to them?’ Since all the pathways share certain fundamental realities, like ‘the main problem is fossil’ , it’s easy to conclude that the main mitigation has to be ‘fossil’.

  12. That is a lot of wisdom and insight crammed into a pretty short post, Fergus. Thanks for sharing that.

  13. Bob Loblaw says:

    Something missing in this discussion of scenarios is the question of “how do you compare different models?” All models consist of inputs and outputs. When comparing outputs, you want to maintain some sort of controlled experiment where the differences between models are due to internal factors, not inputs. To do that, you need to standardize the inputs as much as possible.

    For climate models, obvious common inputs are:
    1) the same solar radiation vs time.
    2) the same atmospheric composition vs time (CO2, etc. – but not things like water vapour or clouds, which are calculated internally)
    3) the same ground cover (if not calculated internally)
    4) etc.

    If you tried to compare two models that used two different solar irradiance reconstructions for the past century, you’d have a hard time telling whether the different outputs are due to different inputs or different internal structures such as cloud, surface albedo, etc.

    In addition, you want to try to have a range of input values that represent a complete set of possible inputs – including the extremes. That means multiple scenarios. The goal here is to exhaust the parameter space of the model.

    And what seemed a reasonable extreme 20-30 years ago (RCP 8.5?) is still a reasonable choice now, even if it is much less probably – simply because you want to be able to compare current models to the behaviour of earlier models that have been run using the scenario.

  14. Fergus,

    I don’t think the pathways/scenarios are ‘what ifs’ so much as simply ‘if thens’.

    That’s a fair point. Although, I was trying to distinguish slightly between the scenario, and how it is then used in some model. The latter seems more “if then”, but maybe “what if” then doesn’t really describe what I was regarding as the scenario all that well anyway.

    Since all the pathways share certain fundamental realities, like ‘the main problem is fossil’ , it’s easy to conclude that the main mitigation has to be ‘fossil’.

    Indeed, it’s not difficult to work out what the fundamental problem is. Actually solving it is trickier 🙂

  15. Bob,
    Yes, that’s a good point. Some scenario use it to test/compare models, rather than to directly inform policy.

  16. cit izen says:

    “Economic forecasting is game theory, which is intractable to model.”

    That seems overly general and harsh.
    Given that any scenario/what if, is going to involve fossil fuel use it is inevitable that producers/suppliers are going to use all and any means to seek a ‘Nash Equilibrium’ that maximises their turnover and profits while minimising any change in the status quo.
    I suspect this eliminates a lot of the current scenarios and can explain a lot of the resistance to action on mitigation to date.

  17. “Nash Equilibrium”

    Game theory encompasses much more than that and includes all of the human decision making that may be irrational. This is particularly apparent from the irrationality of a significant fraction of the population to getting vaccinated or refusing to wear a mask. I don’t think that was incorporated in any of the pandemic modeling. In terms of forecasting fossil fuel usage, Jevons Paradox and the Tragedy of the Commons may be considered as aspect of game theory directly connected to the inability to anticipate human behavior.

  18. izen says:

    “In terms of forecasting fossil fuel usage, Jevons Paradox and the Tragedy of the Commons may be considered as aspect of game theory directly connected to the inability to anticipate human behavior.”

    Actually the Tragedy of the Commons is extensively analysed in game theory, see :-

    But the point is not to predict human behaviour, but to model the behaviour of the large organisations that are the major players in the economics of energy use. Despite US law claiming the contrary corporations are not people. They are neither rational or irrational, but simply seek market share, turnover and profit.

  19. the consumption trap really captures a lot of human beings. It’s like fly paper, we are really stuck to it. Very hard to get free of it.

  20. Have no problem with that. It’s the equivalent of the greedy algorithm in computer science which is to make the locally optimum decision at each step. As oil is the fuel that drives all human consumption it makes sense to obtain as much as one can in the cheapest way possible. Which means that nothing is considered out of bounds, and acceleration in extraction only abated in the 1960’s.

  21. izen says:

    “As oil is the fuel that drives all human consumption it makes sense to obtain as much as one can in the cheapest way possible.”

    For the organisations that actually control consumption the motivation is to sell as little as possible for the highest price. Competition is the only function that restricts this.
    If fossil fuels were regulated by a single authority, it could reduce consumption by exploiting this factor, thereby making alternative energy sources more competitive. But for some reason this is resisted by the corporations involved.

  22. Look how the global consumption curve up until the 1960’s showed an accelerated growth. Oil was used very liberally including spreading it on dirt roads and parking lots to keep the dust down. Then the oil shocks of the 1970’s occurred and the production slowed way down. In hindsight if that accelerated pace had continued, we would be relying on alternative energy sources by now.

  23. PP said “In hindsight if that accelerated pace had continued, we would be relying on alternative energy sources by now.”

    I think you are suggesting that if we had burned through fossil fuel reserves, we would now have a greener energy structure because those reserves would be gone and change would have been required. If so, don’t you think this is a wildly inaccurate supposition because of the carbon loading to the environment that would have happened with that level of fossil fuel consumption?

  24. The implication is that there was a transition in which oil started to be treated as an existential resource and that we have been on a glide path for some time. In the USA there’s much ignorance on how much we produce versus export leading to the idea that we are a significant net exporter of fuel. The USA burns 20 million barrels/day but only extracts about 12 million barrels/day. That means that we are far from being self-sufficient, yet this charade is promoted because energy accountants claim that crude oil imported for the purposes of being further refined counts as “USA oil” if it is re-exported. So that’s all just an accounting trick.

  25. Dave_Geologist says:

    Paul, in a counter-factual world where a combination of political instability, increasing production costs, and a real-if-not-good-enough effort to decarbonise and make more efficient use of what we do burn had not led us to move away from oil, gas and coal (although that got cheaper in real terms as the world moved from deep-mining to strip-mining), we’d be living in a much hotter world with much worse weather excursions, and have had less time to adapt than in the current situation where we’ve deferred that world for a decade or three (depending on how well we implement our Paris pledges*).

    For me and I suspect for most people that would be a glass-half-empty world, and it would take more than accelerated adoption of alternatives when the “Oh Shit!” moment finally dawned to make it half full.

    * Net-zero commitments could limit warming to below 2°C (if both near- and long-term 2021 firm and conditional commitments are fully implemented – in practice I think some negative emissions stuff is pie-in-the-sky and need to be substituted).

    Realization of Paris Agreement pledges may limit warming just below 2 °C (5%–95% range 1.4–2.8 °C).

    Non-paywalled read-only version.

  26. Not sure what you are trying to say here DAVE_GEOLOGIST. We are fortunate that crude oil in the USA was not as plentiful as we first thought, as that acceleration of crude oil usage leading up to the 1960’s would likely to have continued for some time. It’s a thought experiment, sure, but it’s an interesting hypothetical scenario to consider. (BTW, in the 1950’s, MKH predicted max USA production would occur in ~1971)

  27. Dave_Geologist says:

    And like you, Paul, MKH was wrong. Except in the trivial-but-true sense of individual basins depleting, which everyone knows including the oil companies. Including in the 1950s. That’s why they walk away from depleted basins and don’t flog a dead horse. That’s why they walked away from Pennsylvania a century ago.

    But we’ve ridden that rodeo before, and Inconvenient Truths didn’t change your mind then, so I won’t waste my time trying again. Other than to point out that the oil shocks of the 70s and 80s (not resource-driven, except in the sense that a political decision to embargo Western customers stifles delivery of resources, as does OPEC market manipulation) lasted only a few years, and maxed out at about 15% of production after which the trend was relentlessly upwards, with plateaus or small falls in production (but large falls in price) during recessions. Not exponential, but then there was decarbonisation driven by climate not resource arguments (and clean-air regulations), and you can only industrialise China or Indonesia once.

    My point though was that even if we had continued with exponential growth (interrupted by those embargoes and recessions of course), and even if you were right about us depleting a far more limited resource which would have run out by now, forcing us to go big on wind and solar, the world would have been in a much, much worse place by 2022. An RCP8.5 world or worse. Actually, given how storm, fire and flood are biting into individual consciousness these days, I rather think that in that counterfactual world we’d have implemented decarbonisation anyway by now, because public opinion wouldn’t stand for anything else. The Great Fire of 2005, the Great Floods of 2010 and the Crop Crisis of 2015 would have concentrated minds.

  28. Dave said: “… and don’t flog a dead horse”

    That’s an apt description of the current crude oil predicament in the USA, despite cries to the contrary of plentiful untapped reserves. My co-author Dennis Coyne is monitoring the situation as he maintains the POB blog.

  29. Dave_Geologist says:

    Only in the eyes of those who know nothing about the industry, its resource base, its financing, its history, and its geology, geochemistry, geophysics, petrophysics and engineering (which includes a lot of the dentists, hedge funds and private equity who jumped in in ignorance and got their fingers burned, but excludes the professionals who work in the industry, or who don’t but who have the professional skills to understand it and who took it upon themselves to learn about it).

    Its current predicament is over-investment and over-production (although given events to our east that may soon become an advantage not a predicament), and that investments made at a record-high oil price with record-high development costs went underwater when the price collapsed after the Crash. Guess I should have added economics to that list.

  30. angech says:

    Most disappointed.
    Socially very important post on the Ukraine war.
    Please put it up.

  31. angech, I don’t know what you’re getting at.

  32. You can look at the numbers. The USA decides to export 3 million barrels of oil per day, even though they extract only 11 million per day while the country burns 20 million barrels per day.

    In 2012, the NY Times wrote that the USA “will become a net oil exporter by 2030, according to a report released Monday by the International Energy Agency.”

  33. Dave_Geologist says:

    Why should I look at numbers which are totally irrelevant to my response, Paul? Although I should have made clear that my economics lesson was primarily about gas, which is (was) stranded for lack of US LNG expert infrastructure. Shale oil is very much a niche play which rides on the back of gas and is a tiny fraction of shale investment and production. And ironically is the part that was making money while gas prices were bid down to less than the cost of production. And please don’t mention negative Henry Hub prices. That was a distribution bottleneck exacerbated by storage shortages and is totally irrelevant on any timeframe longer than a few weeks. Which I’m pretty sure is not the sort of scenario we’re discussing

    And why should I care about the NYT or Denver Post? I guarantee they’re not knowledgeable or reliable sources. Of course just as future climate projections have to be tied to emissions, future production projections have to be tied to future price and demand scenarios (which are of course interlinked, partly though regulation, which is all factored into the BP and IEA projections which I’d rank above the NYT’s). And I’ve seen precisely zero claims of plentiful untapped (US) reserves (I’ll assume you meant resources). At least from credible sources. Although of course you have to define “plentiful”. Plentiful enough to keep ExxonMobil and the drilling companies happy for decades is not the same thing as plentiful enough to make the US a net exporter for decades, but both fit the imprecise alleged claim. Nor is it the same as the more topic-relevant “plentiful enough globally to burn our way to RCP8.5 if we’re not careful”.

    We have essentially no untapped Reserves, in the USA or elsewhere. Reserves is one of those economic things. They have to be economically viable, and they don’t even make it out of company-internal lists into SEC Reserves until they’ve been flow-tested, and don’t make it into the important category everyone in the know looks at until they’ve been put into commercial production, zone-by-zone too, not whole-field-at-once. 99% of commentators treat SEC Reserves, reserves and resources as the same thing. They’re not. Chalk and cheese. I know because I’ve been responsible for SEC Reserves submissions. You need a course and certification to do so, and you damn well better learn the difference (and between internal company reserves* categories and SEC Reserves, which excludes much of the former which are for internal use only).

    * The main difference is that internal “undeveloped” categories can make future price and engineering-cost projections, which is of course essential for corporate forward planning, but the SEC numbers are based purely on the real-time price and real-time costs in the reported year, and are purely for already-developed resources).

  34. Dave_Geologist says:

    To continue the economic theme, lots of places have lots of (potential) resources, given the right price. At $1000 per barrel the West Lothian oil shales left over from when mining was abandoned due to undercutting by drilled oil would be economic again. Because oil is internationally traded and easily transported, it would be (economically) stupid to develop oil domestically that is more expensive than the world price. Your customers wouldn’t buy your oil, they’d buy Saudi oil. There may of course be energy security reasons to do so, but if the US government wants that it will have to pony up.

    In a scenario where oil really is running out soon, and is not just impacted in price by post-Covid recovery and Russian supply issues (and if buyers are smart, anticipation of Russian supply outages when the GE and Rolls-Royce gas turbines they rely on fail for lack of spare parts – unlike in Soviet days they’re no longer home-made), the price will be bid up and people will be looking at stuff that costs $100, $200, $300 etc. per barrel to develop.

    If Ukraine is smoothed over, the Saudis keep producing and Venezuela comes back into the fold, the price will fall again and those resources will remain undeveloped. Not because they’re not there: because they’re uneconomic at the current and expected price. If oil never gets above $100 per barrel because cheap renewables bid the energy price down and transport is electric-everything, those resources will remain undeveloped. Not because they’re not there: because they’re uneconomic at the current and expected price. In neither case is the world not using them because they don’t exist: it’s not using them because other stuff is cheaper. RCP8.5 will have been averted, but not because there isn’t enough fossil carbon to burn. It will have been averted because we developed cheaper alternatives and only the really cheap stuff like onshore Saudi and Russian supplies can compete. In which case, well done us. But it would be an Act of Man (and Woman), not an Act of God or a Law of Nature.

  35. I believe that the IPCC scenarios are too !narrow!. None of the many scenarios include, however, a steady state or shrinking economy. In the scenario with the lowest increase of the economy, SSP3, the GDP per capita will still double to 2100. This scenario also includes increased inequality and regional rivalry, as if those where necessarily linked to each other.
    The scenarios of the IPCC are not normative but are there to enlighten policy and show possible paths of development. The worst case scenario, SSP5-8,5 shows how the climate will develop if emissions continue and few countermeasures are taken. It shows a global temperature 5 centigrade higher than today. With that in mind it is very hard to understand why there are no IPCC scenarios without growth? After all we all know that 1) endless growth is impossible and 2) the only periods of reduction of emissions are associated with no growth.

  36. Gunnar,
    That has been a reasonably common recent common criticism of the IPCC reports. My understanding is that the latest WGIII report did discuss degrowth scenarios. Some of this is summarised here.

  37. “Not because they’re not there”

    Dave, They’re not there, remember finite & non-renewable? You’d be welcome over at the POB blog, where one of the semi-regular commenters calls himself ReserveGrowthRulz. It’s really about connecting the dots and piecing everything together from incomplete info.

  38. Dave_Geologist says:

    Arithmetic lesson required I see, as well as all those others.

    100% is finite. Back before drilling 1% was technically extractable. The other 99% didn’t magick into existence with Colonel Drake. It was there all the time, but not extractable. Today, technically, we’re in the low tens. Where it sits in those low tens depends on the price, which depends on investment and production more than on what’s there. In the short term, because that investment and production has a 5-10 year lead time, the price depends on demand fluctuations (2008 or Covid, anyone?) and supply fluctuations (wars, boycotts, OPEC). Increasingly, it depends on competing energy sources which likely means the Oil Age will go the way of the Stone Age: it will still be there, but we wont extract it because we don’t need it and it would be silly to extract stuff we don’t need when there are cheaper alternatives.

    If you equate my position to ReserveGrowthRulz*, then you haven’t been reading my posts.

    It really, really isn’t about connecting the dots and piecing everything together from incomplete info. That way lies N-Rays, Aether, Humors, Bile, Caloric, Cold Fusion, Memories of Water (radio transmittable of course) and faster-than-light-neutrinos. Because, then, there’s not just physics, there’s chemistry, geology and its sub-branches, engineering ditto, and economics ditto. Unlike the Greek-to-19th-Century authors of those fallacies, but like the later ones, we have the resources to work with more complete info. I’ve tried in vain to remedy your information deficit so you don’t have to rely on numerology, but there we go.

    Consider this: if you’re right, we don’t need the IPCC, Paris, targets, carbon taxes, subsidies and all the rest. We’ll run out before we wreck the planet. The rest of the world, not just me, disagrees with you. Why? Maybe they know something you don’t know. Or maybe, like me, they realise that even if you are right, we can’t be 100% certain of that and if you’re wrong, it’s Burn Baby, Burn, and Cook Planet, Cook.

    We need a new proverb. The sighted man who refuses to open his eyes and see the elephant, shuts his ears when another sighted person describes the elephant, and insists it’s a rope because by also refusing to accept advice on where the elephant is, he found his way by nose to the smelly end.

    * Except in the technical sense that companies have internal reserves categories that don’t make it into SEC Reserves but are progressing through the development pipeline. That gives a hidden but real 10-20 years of growth as capital investment is sanctioned. Of course there may be a net decline if those pipeline-reserves fall below production of visible Reserves.

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